Spectrum Analyzers produce a display of amplitude vs. frequency of an input signal over a user settable frequency range. Typically, the user sets a center frequency and the unit scans over a predetermined band around the center frequency. Also, an amplitude reference is selected.
In previous spectrum analyzers, measurements of different relevant points on the displayed output was difficult to obtain accurately. In contrast, the preferred embodiment of the present invention provides a moveable marker which may be positioned on and moved along the displayed waveform. The frequency and amplitude at the present position of the marker is displayed and updated as the marker is moved. This moveable relative marker may also be used to facilitate the calculation of frequency and amplitude difference between any two selected points on the display. The marker is positioned on a first point. In response to a control button, a second marker is also provided at that point. This second marker image is then moved to the second measurement point. The difference in frequency and amplitude between these two relative positions is read out directly on the display screen itself.
The amplitude and frequency values at the marker position may be used to adjust various parameters in the machine. For example, the marker may be moved to a point on the display and that point may then be entered as a new center frequency, or it may become the amplitude reference level, or it may become the time interval for center frequency stepping, or using both markers, the interval between them may become the span-width for the entire display, i.e., the two marked positions become the start and stop frequencies.
In previous spectrum analyzers, it was difficult to select a particular portion of the displayed signal and then expand about that point as the center frequency. In the preferred embodiment, the marker may be positioned at any signal point, and using step-up and step-down keys, the frequency span-width about that point may be reduced or expanded.
To maintain the selected signal in the center of the display in previous spectrum analyzers, it was necessary to continuously tune the center frequency to coincide with the desired signal to be viewed as the center frequency. In the preferred embodiment, at the end of each scan, the marker may be positioned to coincide with the peak of the signal then being displayed. The frequency of this peak signal is then set to the center frequency, i.e., the center frequency is set to the value of the peak signal. This eliminates the need to reposition the marker as the scale is expanded since mirror inaccuracies in positioning the marker are corrected during each expansion of the display. Therefore, as the span is reduced, the signal will remain in the center of the display. Also, if the signal is drifting, this feature will maintain the signal on the screen despite the drift.
Prior spectrum analyzers using rotating controls such as potentiometers and other tuning knobs have typically had fixed sensitivities for changing the control parameter in response to a rotation of the same magnitude, since the sensitivity of these controls was usually designed to accommodate the medium range positions. Therefore, the controls would be too sensitive to comfortably tune the lower bands and would move too slowly when tuning over larger bands. Prior spectrum analyzers have used multiple knobs or complicated clutch mechanisms with gear drives to overcome this problem. In contrast, the preferred embodiment; the sensitivity of the rotating control is controlled by the span-width selected, hence, regardless of the frequency range being displayed, a single rotation will still move the marker or other parameters being displayed the same distance across the display.
Previous spectrum analyzers used rotary controls having detent positions or continuously variable tuning. Each function on the spectrum analyzer was assigned either a detent rotary switch or a continuously variable switch. This had the disadvantage that this was not the optimum mode of entry for values for the most convenient way to enter values into the machine. In contrast in the preferred embodiment, a triple entry mode for data input is provided. Using this system, the parameter to be acted upon, e.g., the center frequency, frequency span, the start or stop frequency, etc. is selected, then the value is entered via a keyboard, step-up and step-down keys, or a rotary tuning control. This has the advantage that one may select the most convenient way to enter values for the mode he is operating in. The rotary knob is most useful when tuning or searching for a signal or specific frequency. The step-up and step-down keys are most convenient when one is progressing forward or backward through a limited set of even increments over a range. Of course, the keyboard is most useful for entering the desired value when the exact parameter is known.
In prior spectrum analyzers, if the input signal was preceded by a preamp, a down converter, an up converter, etc., it was necessary to calculate the actual frequency and amplitude for the signal of interest. In contrast the preferred embodiment provides enterable offsets whereby the frequency or amplitude levels can be offset a predetermined amount to calibrate them to take into account the presence of the intervening circuit element.
Also, previous spectrum analyzers were limited to entering a center frequency and span values. In contrast, the preferred embodiment enables the user to input a start and stop frequency and the center frequency and span are automatically determined. In addition, one may enter a new start or stop frequency to increase or decrease the span without having to calculate a center frequency for the band of interest.
In a spectrum analyzer with a digital display, i.e., where the display is produced in response to data representing a discrete number of sampled points, there is the problem that there may not be enough sample points to properly describe the analog signal. The real solutions would be to increase the sampling rate and the number of points displayed, but this is costly and not practical. Other solutions are compromises which sacrifice some information in order to preserve that information which is most useful for spectrum analysis. The preferred embodiment ensures the display of the most important parameter, i.e., correct signal amplitude. It is obtained from a peak detector which precedes the A/D converter. The peak detector holds the max. signal which occurs between samples for the A/D. After sampling the peak detector output, the peak detector is re-set to the input signal level. Thus, no signal peak is lost. Also, the preferred embodiment employs both a positive and negative peak detector circuit to allow a better presentation of signal noise, i.e., give an indication of the amplitude and frequency excursions in the noise signal.